Arc furnace control system



June; 30;, c. w; MORRIS;

ARQ FURNACE: common SYSTEM 2 Sheets-Sheet 1 Filed Deon 2.6,, 1960:

. INVENTOR CLIFFORDVLMORRIS June 30, 1964 c. w. MORRIS 3,139,473

ARC FURNACE CONTROL SYSTEM Filed Dec. 23, 1960 2 Sheets$heet 2 INVENTORCLIFFORD W. MORRIS ATTORN United States Patent 3,139,473 ARC FURNACECONTROL SYSTEM 7 Clifford W. Morris, Modesto, Califl, assignor to FMCCorporation, a corporation of Delaware Filed Dec. 23, 1960, Ser. No.77,913 9 Claims. (Cl. 13-13) This invention relates to a control systemfor electric arc furnaces and particularly to an arrangement wherein thefurnace electrodes are moved with respect to the crucible material tomaintain a substantially constant electrode tip-to-charge resistance.

One use for which the invention has been found especially useful is inconjunction with barium oxide. In the reduction of barium carbonate tobarium oxide in an electric arc furnace, it is required that theelectrode tip be positioned with respect to the crucible material inorder to furnish substantially constant resistance between the tip andthe material. With substantially constant resistance, the powerdelivered to the crucible material by the arc to elevate the temperatureof the material Will be maintained substantially constant.

Barium carbonate at temperatures beyond the eutectic one and near theconversion temperature bubbles and boils with a rolling and splashingaction in the crucible. Therefore, a sensitive and fast acting controlsystem is required in order to maintain even a semblance of constantelectrode tip-to-charge resistance and consequently constant currents toprovide a uniform delivery of heat. If the control does not respondrapidly enough, the electrode movement may lag far behind the resistanceand current changes with the result that an oscillating condition is setup accompanied by alternately moving of the electrode, resulting in aheavy over-current condition ertial'problems.

One of the objects of the present invention is to provide an arrangementfor rapidly moving the furnace electrodes with respect to the cruciblematerial to maintain 'uniforrn heating of the material.

A further object of the invention is to provide a means for controllingthe position of the furnace electrodes with respect to the materialin acrucible or the like with provisions for readily adjusting the controlsystem for changes in the material and the temperature to which it is tobe subjected.

A still further object of the invention is to provide means forcontrolling a hydraulic actuator for moving the electrodes with highsensitivity and rapid response.

In one aspect of the invention the control system is employed with anA.C. electric arc furnace having movable electrode means. The furnacemay be single phase or polyphase. Motor means are provided to. raise andlower the electrodes, said motor means preferably being a double acting,hydraulically operated servo motor. Means are provided for producing afirst control signal in response to the current flowing to the electrodemeans. Other means are provided for producing a second control signal inresponse to the voltage across the electrodes and furnace. A saturablereactor is included which has gate windings connected to both the secondcontrol signal and torque motor or similar arrangement for operating ahy- H draulic valve controlling the servo motor. The control windingmeans of the saturable reactor provides a predetermined bias to thereactor and in addition a feedback or control connection from the firstcontrol signal circuit is made to thecontrol winding in an opposingrelationship to the bias so that an increase in electrode current flowwill tend to increase the gate winding impedance and thereby decreasethe signal fed to the differential control means. The bias also may beconnected so as to accentuate the second control signal.

These and other objects, advantages and features of the invention willbecome apparent from the following description and drawings.

7 In the drawings:

FIG. 1 is an elevational view of the electric arc furnace and the meansfor positioning the electrodes with respect to the crucible material;and

FIG. 2 is a schematic representation of the control circuit for thepositioning of the electrodes.

In one form of furnace with which the invention is to be used, anelectric furnace 10 (FIG. 1) includes shell 11 in which is disposed acharge 12, such as barium carbonate, which is to be reduced at hightemperature. Other materials, of course, can be treated. Duringoperation, a portion of the partially converted charge remains solid andacts as a crucible. For convenience in handling, shell 11 may be mountedupon car 13 which is adapted to travel along rails 14. Furnaceelectrodes 15 are mounted in electrode stem 16 Which is supported bycable 17. Cable 17 is operated by cylinder assembly or servo motor 18which is located adjacent to electric furnace 10.

The power source of servo motor 18 is hydraulic cylinder 19 whichpositions cylinder rod 20 in a linear manner. Hydraulic cylinder 19 issupported with respect to base plate 21 by means of uprights 22. At thelower end of cylinder rod 20, cross head 23 is attached by means of pin24 and coupling 25. Cross head 23 is adapted to be guided in itsverticalmotion by engagement with uprights 22. At the bottom portion ofcross head 23 shaft 26 is mounted by means of bearing 27. Shaft 26serves as a pivotal support for moving sheave 28 over which cable 17extends.

One end of cable 17 is attached to eye bolt 29 which is mounted uponsupport 30 of cylinder assembly 18 by means of spring 31 and plate 32.Cable 17 after passing about moving sheave 28 extends across sheave 33and sheave 34 onto the connection with stem 16. It is evident that withone end of cable 17 attached to servo motor 18, motion of the cylinderrod and the moving sheave driven by it will cause the cable to move andthereby raise or lower the electrode stem and the'electrode orelectrodes extending from it. Limit switch 35 mounted upon one of theuprights engages cross head 23 at the lower extent of its stroke andterminates its. downward movement, thereby establishing an up-limit forthe elec trodes. Switch 36 mounted upon support 30 is actuated by plate32 whenever cable 17 becomes slack and permits spring 31 to expand in anupward direction. This switch serves to safeguard the equipment wheneverthe electrodes are. lowered to a point where they bottom upon the chargeor some other obstruction.

Control system 37 (FIG. 2) is adapted to operate servo motor means 18 tomaintain a substantially constant resistance condition betweenelectrodes 15 and charge 12 disposed within shell 11. Power is deliveredto electrodes '15 by means of line 38 which is connected to anelectrical power source (not shown). The electric circuit from electrode15 continues through the are between the elec- 'trode and the charge andthen through the grounded shell. Control system 37 operates hydraulicservo valve 39 which in turn controls the flow of hydraulic fluid underpressure to either end of hydraulic cylinder 19 in order to raise orlower the electrodes. In FIG. 2, the ports of the valve cylinder aresymbolically represented at 40 which are controlled by valve spool 41 inthe usual manner. The servo valve includes a torque motor comprisingpole piece 42, as well as Raise coil 43 and Lower coil 44. The coils areconnected to one another at terminal 45. The electromagnetic fieldsinduced in pole piece 42 by the differential currents across the coilsrotate torque armature 46 about its central portion 47 soas to movespool 41 which is pivotally connected to the armature. Valve spool 41 isdirectly actuated by the torque motor.

Current transformer 48 translates the varying current in line 38 into ausable voltage signal or first control signal which substantiallyduplicates the current signal in amplitude and frequency. The voltagesignal from current transformer 48 is applied across loading resistor49, one end of which is grounded.

The operators control panel 50 for control system 37 has three positionsdesignated A, B and C corresponding to Lower, Automatic and Raise modes.Control panel 50 includes switches 51, 52 and 53 which maybesimultaneously set at three positions, a, b and c. In the Automaticmode, the voltage of loading resistor 49 is applied to the primarywinding 54 of isolation transformer 55. The connection is made throughline 56, switch 51, terminal 51b, normally closed relay contacts 57, andline 58 to primary winding 54. The voltage signal induced in secondarywinding 59 of isolation transformer 55 is then connected to furnacecurrent adjusting potentiometer 60, one end of which is grounded. Slider61 of potentiometer 60 connects the signal to the bridge rectifier 62,the output of which is connected to lines 63 and 64. Line 63 is conectedto one end of Raise coil 43 while line 64 is connected through line 65to terminal 45 of the Raise coil. In this way, a DC. voltage, being afunction of the electrode current, is applied to the Raise coil with thenegative polarity at terminal 45. Potentiometer 60 is adjusted toestablish a predetermined DC. voltage across the Raise coil 43 for apredetermined current to electrodes 15, but it does not limit the peaksof current signals impressed upon the coil.

The voltage of electrodes 15 with respect to ground of the arc voltageis connected by means of line 66 to primary winding 67 of isolationtransformer 68. In the Automatic mode, the arc voltage is impressedacross the upper portion of primary winding 67 which leads to line 69,terminal 52b, switch 52, line 70, normally closed relay contact 71 andline 72 connected to tap connector 73. The voltage may be adjusted byselectively placing tap connector 73 in circuit with terminals 74 or 75.

The output of isolation transformer 68 is connected to the commonterminal of gate windings 76 and 77 of magnetic amplifier or saturablereactor system 78. When the alternating signal connected to the gatewindings is positive, a circuit is completed through gate winding 77,rectifier 79a, line 80 and line 65 to terminal 45 at the servo valveoperator. The return path of the circuit is through Lower coil44, line81, rectifier 79b and line 82 to ground. For a negative going signal,the cricuit extends from ground, line 83, rectifier 790, line 80, line65 to Lower coil 44. The return path is through line 81, rectifier 79d,to gate winding 76. Bias for the saturable reactor or magnetic amplifier78 may be provided by means of potentiometer 84 connected acrossterminals 85 and 86 adjacent to the rectifiers which energize b-iaswinding 87. Potentiometer 84 is adjusted so as to energize bias winding87 and thereby bring to a minimum the high inductive impedance ofmagnetic amplifier 78 so that a practically unimpeded and uninfluencedsignal may reach lower coil 44. Magnetic amplifier 78 is provided with asignal, which is in effect a feedback signal, to control winding 88which is energized by a circuit extending from line 63 throughpotentiometer 89 and line .90. The magnitude of the feedback or thirdcontrol signal is determined by the setting of potentiometer 89.

With increasing current through line 38 to electrodes 15, the currentsignal appearing across lines 63 and 64 of bridge rectifier 62increases. This results in an increased signal being connected from line63 to control winding 88 of saturable reactor 78 which opposes thesignal in bias winding 87. As a result of these opposing signals, theoverall impedance of saturable reactor 78 increases so that the outputvoltage across terminals and 86 is reduced, thereby reducing the voltageapplied to Lower coil 44. At the same time, the increased current signalfrom bridge rectifier 62 has caused an increased signal across Raisecoil 43 with the result that the combined coil signals double the torqueresponse applied to armature 46. The combined effect of the coil signalscauses a large response in servo valve 39 with the result that thecontrol rapidly positions hydraulic cylinder 19 to restore the desiredoperating conditions.

When the arc current decreases and the voltage across the arc increases,the voltage signal applied to primary winding 67 of isolationtransformer 68 increases. At the same time, current transformer 48delivers a reduced current signal to bridge rectifier 62. Consequently,the signal delivered from line 63 to control winding 88 of magneticamplifier 78 is also reduced. The reduced energy of control winding 88in opposition to that of bias winding 87 results in an increase in thesaturation of magnetic amplifier 78. This causes a decrease in theimpedance of the magnetic amplifier so that a greater proportion of theincreased arc voltage signal coupled from isolation transformer 68 isdelivered at terminals 85 and 86 of the magnetic amplifier. In turn, thegreater proportion of the increased arc voltage is impressed acrossLower coil 44 of servo valve 39. Thus, with a decrease in current at thearc, a decreased voltage is applied to Raise coil 43 and an increasedvoltage is applied to Lower coil 44 so that the torque response ofarmature 46 is effectively doubled to drive the servo valve to lower theelectrodes.

Thus, it may be seen that the control system is capable of increasingthe difference between the voltage and current produced signals reachingthe servo valve in aproportional manner and also having the differencesignal returned to a zero value at a rate proportional to that at whichoriginal signal differences at the electrode return to zero. In thisway, the control system is capable of taking advantage of the highfrequency response characteristics and the load signal differentialthreshold of the torque motor in servo valve 39.

The operators control 50 enables the operator to select the Lower,Automatic and Raised modes of operation. Control 50 is energized fromA.C. source 91 which is connected to primary winding 92 of transformer93. In the Automatic mode, switches 51, 52, 53am set in the B position.In the Lower mode, switches 51, 52, 53 are set in the A position. In theA position, switch 51 by means of line 56 grounds the current signalfrom current transformer 48 to terminal 51a. At the same time, switch 53connects secondary winding 94 of transformer 93 through switch 95, line96, relay contacts 71 and line 72 to tap connection 73. This results inan increased signal being applied to magnetic amplifier or saturablereactor means 78 with the result that an increased signal is impressedacross Lower coil 44 of the servo valve. In this manner, the electrode15 can be manually directed to descend toward charge 12 in shell 11.

To establish the Raise mode, switches 51, 53 are set in the C position.Switch 51 again grounds the output of current transformer 48. Switch 53connects secondary winding 94 through switch 95, relay contacts 97 andline 58 to primary winding 54 of isolation transformer 55. In this way,an increased signal is applied to the isolation transformer andultimately through the bridge rectifier to Raise coil 43 of the servovalve. As a result of this, servo motor 19 elevates the electrodes awayfrom the crucible.

motor may" have a resistance of 70 ohms.

Merely by way of example, the servo valve torque The current circuitscan be set to provide nine volts D.C. across one torque motor windingwhen 3,000 amperes flow in the electrode circuit. The Voltage sensingcontrol circuit can be adjusted to furnish nine volts DC. to the othertorque motor coil at 150 volts A.C.

If less sensitivity is desired, a fixed bias can be supplied to thesaturable reactor in place of the bias furnished by winding 87 which isdependent upon the voltage appearing across the gate windings.

It should be apparent that variations can be made in the construction asneeded without departing from the spirit of the invention except asdefined in the appended claims.

What is claimed is:

1. In a control system for electric arc furnaces having electrode meansconnected to a source of electrical energy, the combination includingmotor means connected to said electrode means for raising and loweringthe same, means producing a first control signal in response to thevoltage across said electrode-means and furnace, saturable reactor meanshaving gate winding means connected to said second control signal and todifferential control means for said motor means, means connecting saidfirst signal to said differential control means, control winding meansfor said saturable reactor means providing a predetermined bias thereto,and a feedback connection from said first control signal to said controlwinding means connected in opposing relationship to said predeterminedbias, so that increase in current flow will tend to increase impedancein said gate winding means and thereby decrease the signal fed to saiddifferential control means, so that changes in current and voltagerelations in said electrode means will be accentuated and therebyrapidly adjust said electrode means.

2. In a control system for electric arc furnace means having electrodemeans connected to a source of electrical energy, said electrical energybeing adapted to form an are at said electrode means, the combinationincluding means for raising or lowering the electrode means with respectto the furnace, means for producing a first control signal in responseto the current'flowing in said electrode means, means for producing asecond control signal in response to the voltage across thearc,saturable reactor -means having gate winding means connected to saidmeans for producing a second control signal, control winding meansconnected to said means for producing a first control signal, windingmeans for providing bias to said saturable reactor, said control windingmeans and said bias winding means being in opposingrelationship in saidsaturable reactor, and differential control means for said means forraising and lowering the electrode means, said differential controlmeans being connected to said means for producing a first control signaland to said gate Winding means, so that an increase in said firstcontrol signal with increased electrode current will increasetheimpedance of said gate winding presented to the reduced secondcontrol signal accompanying said increased electrode current and therebydecrease the signal fed by said gate winding means to said differentialcontrol means, so that changes in current and voltage relations in saidelectrode means will be accenuated and thereby rapidly adjust saidelectrode means.

3. In a control system for electric arc furnace means having electrodemeans connected to a source of electrical energy, said electrical energyforming an arc at said electrode means, the combination including meansfor raising or lowering the electrode means with respect to the furnace,means for producing a first control signal in response to the currentflowing in said electrode means, means for producing a second controlsignal in response to the voltage across the arc, saturable reactormeans having gate winding means connected to said means for producing asecond control signal, control Winding means connected to said means forproducing a first control signal, winding means connected to said gatewinding means for providing bias to said saturable reactor, said controlwinding means and said bias winding means being in opposing relationshipin said saturable reactor, and differential control means for said meansfor raising and lowering the electrode means, said differential controlmeans being connected to said means for producing a first control signaland to said gate winidng means, so that an increase in said firstcontrol signal with increased electrode current will increase theimpedance of said gate winding presented to the reduced second controlsignal accompanying said increased electrode current and thereby decrease the signal fed by said gate winding means to said differentialcontrol means, so that changes in current and voltage relations in saidelectrode means will be accentuated and thereby rapidly adjust saidelectrode means.

4. In a control system for electric arc furnace means having electrodemeans connected to a source of electrical energy, said electrical energyforming an are at said electrode means, the combination including meansfor raising or lowering the electrode means with respect to the furnace,means for producing a first control signal in response to the currentflowing in said electrode means,

first transformer means connected to said first signal producing means,means for producing a second control signal in response to the voltageacross the arc, second transformer means connected to said second signalproducing means, saturable reactor means having gate windingmeansconnected to said second transformer means, control winding meansconnected to said first transformer, winding means for providing bias tosaid saturable reactor, said control winding means and said bias windingmeans being in opposing relationship in said saturable reactor, anddifferential control means for said means for raising and lowering theelectrode means, said differential control means being connected to saidfirst transformer means and to said gate winding means, so that anincrease in said first control signal with increased electrode currentwill-increase the impedance of said gate winding presented to thereduced second control signal accompanying said increased electrodecurrent and thereby decrease the signal fed by said gate winding meansto said differential control means, so that changes in current andvoltage relations in said electrode means will be accentuated andthereby rapidly adjust said electrode means.

5. In a control system for electric arc furnaces having electrode meansconnected to a source of A.C., the combination including hydraulicallyoperated motor means connected to said electrode means for raising and.lowering the same, first circuit means producing a first control signalin response to the current flowing in said electrode means, secondcircuit means producing a second control signal in response to thevoltage across said electrode means, saturable reactor means having gatewinding means connected to said second circuit means and second controlsignal, differential'motor means having one portion connected to saidgate winding means, means connecting said first signal to anotherportion of said differential motor means, control Winding means for saidsaturable reactor means having an electric bias source connectedthereto, a connection from said first circuit means for feeding a signalto said control winding means in opposing relationa ship to said bias,and means operated by said differential D.C. control signal in responseto the voltage across said electrode means, saturable reactor meanshaving gate winding means connected to said second circuit means andsecond control signal, differential torque motor means having oneportion connected to said gate winding means, means connecting saidfirst signal to another portion of said differential motor means,control winding means for said saturable reactor means having anelectric bias source connected thereto, a connection from said firstcircuit means for feeding a signal to said control winding means inopposing relationship to said bias, and hydraulic valve means directlyoperated by said differential torque motor means connected to saidhydraulically operated motor means for operating the same, so thatchanges in current and voltage relations in said electrode means will beaccentuated and thereby rapidly adjust said electrode means.

7. In a control system for electric arc furnaces having electrode meansconnected to a source of A.C., the combination including hydraulicallyoperated motor means connected to said electrode means for raising andlowering the same, first circuit means having rectifier means thereinfor producing a first D.C. control signal in response to the currentflowing in said electrode means, second circuit means having rectifiermeans therein for producing a second D.C. control signal in responsetothe voltage across said electrode means, saturable reactor means havinggate winding means connected to said second circuit means and secondcontrol signal, differential torque motor means having one portionconnected to said gate winding means, means connecting said first signalto another portion of said differential motor means, control windingmeans for said saturable reactor means having an electric bias sourceconnected thereto, a connection from said first circuit means forfeeding a D.C. signal proportional to said first signal to said controlwinding means in opposing relationship to said bias, and hydraulic valvemeans directly operated by said differential torque motor meansconnected .to said hydraulically operated motor means for operating thesame, so that changes in current and voltage relations in said electrodemeans will be accentuated and thereby rapidly adjust said electrodemeans.

8. In a control system for electric arc furnaces having electrode meansconnected to a source of A.C., the combination including hydraulicallyoperated motor means connected to said electrode means for raising andlowering the same, first circuit means having rectifier means thereinfor producing a first D.C. control signal in response to the currentflowing in said electrode means, second circuit means having rectifiermeans therein and an isolation tranformer for producing a second D.C.control signal in response to the voltage across said electrode means,satu rable reactor means having gate winding means connected to saidsecond circuit means and second control signal, differential torquemotor means having one portion connected to said gate winding means,means connecting said firstsignal to another portion oftsaiddifferential motor means, control winding means for said saturablereactor means having an electric bias source connected thereto,

aconnection from said first circuit means for feeding a D.C. signalproportional to said first signal to said control winding means inopposing relationship to said bias, and hydraulic valve means directlyoperated by said differential torque motor means connected to saidhydraulically operated motor means for operating the same, so thatchanges in current and voltage relations in'said electrode means will beaccentuated and thereby rapidly adjust said electrode means.

9. In a control system for electric arc furnace means having electrodemeans connected to a source of A.C., said A.C. being adapted to form anare at said electrode means, the combination including hydraulicallyoperated servo motor means for raising or lowering the electrode meanswith respect to the furnace and a charge therein, transformer meansassociated with the AC. supply to said electrode means for producing afirst control signal in response to the current flowing in saidelectrode means,

I means connected across said furnace for producing a second controlsignal in response to the voltage across the same, saturable reactormeans having gate winding means connected to said means for producing asecond control signal, control winding means for said saturable reactormeans, means connecting D.C. to said control winding means for providingbias to said saturable reactor, rectifier means connected to saidtransformer means, means connecting the D.C. signal derived from saidfirst control signal through said rectifier means to said controlwinding means in opposing relationship to said bias, and differentialmeans responsive to said first and second signals connected to valvemeans for controlling the raising and lowering of the electrode means,so that an increase in said first control signal with increasedelectrode current will increase the impedance of said gate windingpresented to the reduced second control signal accompanying saidincreased electrode current and thereby decrease the signal fed by saidgate winding means to said differential control means and therebyrapidly adjust said electrode means.

References Cited in the file of this patent UNITED STATES PATENTS2,717,326 Gunton Sept. 6, 1955 2,884,580 Sauter et al Apr. 28, 19592,921,107 Toothman et a1 Jan. 12, 1960

1. IN A CONTROL SYSTEM FOR ELECTRIC ARC FURNACES HAVING ELECTRODE MEANSCONNECTED TO A SOURCE OF ELECTRICAL ENERGY, THE COMBINATION INCLUDINGMOTOR MEANS CONNECTED TO SAID ELECTRODE MEANS FOR RAISING AND LOWERINGTHE SAME, MEANS PRODUCING A FIRST CONTROL SIGNAL IN RESPONSE TO THEVOLTAGE ACROSS SAID ELECTRODE MEANS AND FURNACE, SATURABLE REACTOR MEANSHAVING GATE WINDING MEANS CONNECTED TO SAID SECOND CONTROL SIGNAL AND TODIFFERENTIAL CONTROL MEANS FOR SAID MOTOR MEANS, MEANS CONNECTING SAIDFIRST SIGNAL TO SAID DIFFERENTIAL CONTROL MEANS, CONTROL WINDING MEANSFOR SAID SATURABLE REACTOR MEANS PROVIDING A PREDETERMINED BIAS THERETO,AND A FEEDBACK CONNECTION FROM SAID FIRST CONTROL SIGNAL TO SAID CONTROLWINDING MEANS CONNECTED IN OPPOSING RELATIONSHIP TO SAID PREDETERMINEDBIAS, SO THAT INCREASE IN CURRENT FLOW WILL TEND TO INCREASE IMPEDANCEIN SAID GATE WINDING MEANS AND THEREBY DECREASE THE SIGNAL FED TO SAIDDIFFERENTIAL CONTROL MEANS, SO THAT CHANGES IN CURRENT AND VOLTAGERELATIONS IN SAID ELECTRODE MEANS WILL BE ACCENTUATED AND THEREBYRAPIDLY ADJUST SAID ELECTRODE MEANS.